Implantable leads having electrodes are used in a variety of applications, including the delivery of electrical stimulation to surrounding tissue, neural or otherwise. Some leads include lumens (or channels) for the delivery of other elements, including chemicals and drugs. Whether in a stimulation or element delivery capacity, such leads are commonly implanted along nerves, within the epidural or intrathecal space of the spinal column, and around the heart, brain, or other organs or tissue of a patient.
Generally, several elements (conductors, electrodes and insulation) are combined to produce a lead body. A lead typically includes one or more conductors extending the length of the lead body from a distal end to a proximal end of the lead. The conductors electrically connect one or more electrodes at the distal end to one or more connectors at the proximal end of the lead. The electrodes are designed to form an electrical connection or stimulus point with tissue or organs. Lead connectors (sometimes referred to as terminals, contacts, or contact electrodes) are adapted to electrically and mechanically connect leads to implantable pulse generators or RF receivers (stimulation sources), or other medical devices. An insulating material typically forms the lead body and surrounds the conductors for electrical isolation between the conductors and for protection from the external contact and compatibility with a body.
Such leads are typically implanted into a body at an insertion site and extend from the implant site to the stimulation site (area of placement of the electrodes). The implant site is typically a subcutaneous pocket that receives and houses the pulse generator or receiver (providing a stimulation source). The implant site is usually positioned a distance away from the stimulation site, such as near the buttocks or other place in the torso area. In most cases, the implant site (and insertion site) is located in the lower back area, and the leads may extend through the epidural space (or other space) in the spine to the stimulation site (middle or upper back, or neck or brain areas).
Current lead designs have different shapes, such as those commonly known as percutaneous and paddle-shaped leads. Paddle leads, which are typically larger than percutaneous leads, are directional and often utilized due to desired stimulus effect on the tissues or areas. However, current paddle-shaped leads require insertion using surgical means, and hence, removal through surgical means.
Percutaneous leads are designed for easy introduction into the epidural space using a special needle. Therefore, such leads are typically smaller and more nearly circular in cross-section than paddle-shaped leads. This reduced size facilitates their implantation in the body, allows their implantation into more areas of the body, and minimizes the unwanted side effects of their implantation.
Larger cross-section leads are required, however, when greater numbers of electrodes are employed at the distal end of a lead. Several benefits can be gained from increasing the number of electrodes on a lead. More electrodes of smaller size allow a stimulation pattern to be more precisely localized, reducing unwanted stimulation of nearby areas and minimizing stimulation side effects. With more electrodes available, stimulation patterns can be moved from the electrodes selected during and immediately after implantation to other electrodes on the lead in order to adapt to post-implantation migration of the lead and changes in the body's responsiveness to stimulation. The presence of more electrodes permits adjacent electrodes to be employed in anode-cathode combinations for increased control of the directionality and penetration of stimulation patterns. However, an increase in the number of electrodes results in an increased number of conductors in the lead body and, thus, in leads of larger cross-section.
Smaller diameter conductors may be employed, in order to reduce the lead body diameter, but such conductors have a higher resistivity than larger diameter conductors. As will be understood by one skilled in the art, a higher resistivity conductor produces a greater voltage drop through the conductor, and thus delivers a lower stimulation voltage at the distal end electrode than would be delivered by a lower resistivity conductor. Greater resistivity in the conductor also results in less current being delivered at the stimulation site. More power is lost in the conduction of the stimulation signal through a higher resistivity conductor, and in some cases, requires more frequent recharging of the power source in the implantable medical device.
Rather than adding conductors to the lead body, an embedded controller may be employed at the distal end of the lead to control a greater number of electrodes with fewer conductors. Known techniques of signal multiplexing may be used to implement the embedded controller. Alternatively, microcontrollers may be used, and the desired stimulation pattern communicated from the connector at the proximal end of the lead to the microcontroller at the distal end using digital communication techniques. While such solutions permit a reduced cross-section in the lead body, they require an increased cross-section at the distal end of the lead to accommodate the circuitry required to implement the solution.
The presence of metallic conductors in stimulation leads can cause side effects when magnetic resonance imaging (MRI) is employed, as well. Currents and voltages induced in the conductors by the electromagnetic field of a MRI scanner may result in uncontrolled stimulation of nerves near the electrodes of a stimulation lead or, in extreme cases, in nerve damage. Such results have led the FDA and manufacturers of implantable medical devices to issue a contraindication for the use of MRI with patients with such devices implanted.
Many other problems and disadvantages of the prior art will become apparent to one skilled in the art after comparing such prior art with the present invention as described herein.